The present invention relates system for supplying coating liquid to spray coating apparatus, and in particular to systems for supplying conductive coating liquid to electrostatic spray coating apparatus.
Systems for supplying coating liquid to spray coating apparatus often embody a color changer, particularly in industrial operations where articles are to be spray coated at a station or as they move along a production line. Where the articles are to be coated a wide variety of colon, it generally is not practical to establish separate spray stations or production lines for each color, or even to spray a long sequence of articles one color, then another long sequence of articles a second color, etc. Instead, it is desirable to make color changes rapidly and simply at a single station.
Electrostatic spray coating provides for increased coating efficiency over nonelectrostatic spray coating. In order to charge the coating liquid, in some systems an electrode is connected to a high voltage supply and placed in close proximity to or in contact with the coating liquid either just prior to or very close to the point of atomization of the coating liquid. In rotary atomization systems, the rotary atomizer may be conductive and connected to the power supply, so that the atomizer is the electrode. The charging voltage connected to the electrode is usually on the order of several tens of kilovolts, which does not present a problem in maintaining a grounded primary supply of coating liquid when spraying nonconductive coating liquid. However, when spraying coating liquids that are moderately conductive, precautions must be taken to prevent the high voltage at the spray device from short circuiting to ground through a conductive column of coating liquid leading back to a grounded primary supply.
One early approach to prevent shorting out the high voltage was to not ground the primary supply of coating liquid, but instead to isolate the entire primary supply and any color change system from ground potential. This allowed the entire coating liquid system to "float" at the charging potential, but had the drawback that a large mount of electrical energy was capacitively stored in the system. To prevent the capacitively stored energy from presenting a shock hazard to operating personnel, it was necessary to provide a protective enclosure around the mating liquid system, which increased the cost of the system and required that the spraying operation be shut down and the system electrically discharged whenever necessary to replenish the supplies of coating liquid. Also, during operation of the system, the capacitively stored energy gave rise to the possibility of a spark causing an explosion in an environment such as exists when solvent based paints are being sprayed.
Another prior approach was to ground the primary supply of coating liquid and any color changer, and connect the spraying apparatus to the primary supply through a hose that was long enough that the electrical resistance of the coating liquid in the hose, between the high voltage at the spray device and the coating liquid primary supply, was large enough to reduce electrical current leakage to a level that did not short out the electrode or cause the charging voltage to fall to an unacceptable level. A disadvantage of the approach was that the hose, due to its extended length, not only was very bulky and hard to manage, but also it was difficult to flush the hose clean of one color of coating liquid in preparation for spraying another color. Further, although the extended length of the hose decreased the magnitude of leakage current, leakage current nonetheless occurred and represented "wasted" charging energy.
Another known and more recent technique involves the use of an intermediate storage tank of relatively small capacity that is electrically insulated from ground, at least during spraying, and is adapted to be intermittently supplied with coating liquid for delivery to the spray device. A switchable high voltage source is provided, together with means for separating or at least electrically insulating the intermediate storage tank during electrostatic spraying from parts of the system that are permanently grounded.